I've tried it as a challenge for a couple of days (lynx, mutt, some other TUI stuff) and it made some things like Vim stick (although that may have as much to do with that challenge as Tridactyl did). But I couldn't last longer than a week. It does free you from the burden of system requirements. CPU: Optional.
I imagine your display is almost entirely black for the majority of the time, with your (most probably) LCD backlight blasting away, trying its hardest to get a few thousandths of its light output through the few pixels on the screen that it can escape! XD
And powertop (great piece of software, thanks Intel) tuned to the max + powersave scheduler. All that on Windows or KDE results in about 4-5h of battery though. So fbdev must be somehow really frugal.